Typically, two one inch incisions are made at the corners of the victim's mouth. Then the victim is tortured causing them to scream. This tears the flesh along the sides of the mouth, leaving a "grin".

A Chelsea Grin (also known as a Chelsea Smile or a Glasgow Grin) is an act popular among British gangs which involves making a cut from the side of someones mouth all the way up to their ear. Due to the cut, it appears as if the victim is smiling. Thought to have originated from Glaswegian criminals but became popular among British football hooligan firms, in particular the Chelsea Headhunters.

Dennis: Let's give this cunt a Chelsea Grin!

Darren: Yeah alright, we can use the knife my mother got me for my birthday.

Cunt: Please, no, not the Chelsea Grin, anything but the notorious Chelsea Grin!

a victim has a card e.g a credit card forced inside there mouth, then the attackers punch the victim in the stomach to make there muscles tighten, making the mouth snaller, the card digs into the skin and makes a "grin" at the sides of the victims mouth.